APOPTOSIS

contents:

• Definition of apoptosis
• Causes
• Morphology
• Mechanism
• Disorders
• Differences between Apoptosis and Necrosis

Definition/What is Apoptosis?

It is a type of (planned) cell death induced by tightly regulated suicide program.

In apoptosis, intrinsic enzymes of the cells get activated that degrade and destroy it’s own nuclear DNA and proteins (both nuclear and cytoplasmic)

The literal meaning of the word APOPTOSIS is ‘falling off’, because fragments of apoptotic cells (apoptotic bodies) break and fall off from the cells.

What causes Apoptosis?

Apoptosis takes place in two broad contexts i.e.,

1. In normal physiological situations
2. In pathological situations

Physiological situations: Death of cells by apoptosis is a normal phenomenon that is involved in eliminating cells that are no longer of any use or as a system to maintain a constant number of cell population in tissues.

• Elimination of excess cells during embryogenesis amd developement process. Example; disappearance of web tissues between fingers and toes.
• Elimination of cells after discontinuation of hormonal stimuli. Example; endometrial cell breakdown during menstrual cycle.
• Removal of potentially harmful cells. In immunology, the clones of self reactive lymphocytes that recognize normal self antigens are deleted by apoptosis.

Pathological situations: Cell loss in various pathological States is attributed to apoptosis. It elimitates cells that are injured beyond repair without bringing out a host reaction, thus limiting collateral damage.

• Elimination of cells with DNA damage: Radiation and cytotoxic anticancer drugs can damage DNA , either directly or via production of free radicals. If the repair mechanisms cannot correct the damage, the cells induce apoptosis. Apoptosis has a protective effect by pretending the survival of cells with DNA mutations that can lead to malignant transformation.
• Elimination of cells with excessively accumulated misfolded proteins . There are several degenerative diseases of the Central nervous system (CNS) like Alzheimer, Huntington and Parkinson’s disease where apoptosis takes place due to accumulation of misfolded proteins.
• Killing of viral infected cells: In viral infections, the infected cells are lost mainly due to apoptosis induced either by the virus or by host human response by cytoplasmic T lymphocytes.
• Apoptosis also takes place in cases of tumors and cellular rejection of transplant.
• Obstruction of ducts in parenchymal organs like pancreas, parotid glands and kidney can lead to death of parenchymal cells by apoptosis.

Morphology of cells undergoing apoptosis:

• Cells shrink
• Nuclear condensation and fragmentation
• Cells first show extensive surface blebbing (irregular bulge in plasma membrane of cell) followed by fragmentation into membrane bound apoptotic bodies.
• Phagocytosis of apoptotic bodies

Under light microscope , the apoptotic bodies are round or oval mass having intensely eosinophilic cytoplasm (dark pink).

The nuclei appears as fragments of dense nuclear chromatin and shows pyknosis.

Mechanism :

Apoptosis is regulated by biochemical pathways. The survival or apoptosis of many cells depends upon balance between two oppsite sets of signals. 1. Death signals (proapoptotic)

2. Prosurvival (antiapoptotic)

Apoptosis results from activation of enzymes called as caspases.

There are two phases of apoptosis:

1. Initiation phase
2. Execution phase

1. Initiation Phase:

There are two pathways that initiate apoptosis.

1. Intrinsic or mitochondrial pathway
2. Extrinsic or death receptor pathway

1. Intrinsic or mitochondrial pathway:

Mitochondrial damage is the major mechanism in a variety of physiological and pathological apoptosis. It is activated by intracellular signals. Survival or apoptosis of cell is determined by permeability of mitochondria.

Mitochondrial permeability is controlled by a family of more than 20 proteins. This family of proteins is called BCL2. These proteins may be broadly divided into proapoptotic or antiapoptotic (prosurvival).

• Proapoptotic proteins: BAX and BAK
• Antiapoptotic protein: BCL2, BCL-XL and MCL1. These proteins prevent the leakage of mitochondrial protein that trigger apoptosis.

Causes of mitochondrial injury:

• Deprivation or withdrawal of growth factors or survival signals.
• DNA damage by radiation, cytotoxic anticancer drugs, hypoxia either directly or through free radical.
• Excessive accumulation of misfolded proteins.
• Increased intracellular free calcium.

Steps in intrinsic pathway: Mitochondrial damage leads to increased mitochondrial permeability and release proapoptotic molecules into the cytoplasm.

• Mitochondrial damage activate a number of sensors of BCL2 family called BH3-proteins . They inturn activate two critical proapoptotic BCL2 family effector proteins, namely BAX and BAK.
• BAX and BAK create several channels in the mitochondria which leads to leakage of mitochondrial proteins from the inner mitochondrial membrane to the cytoplasm.
• One of the leaked proteins is cytochrome c which binds to a protein called apoptosis-activating factor-1 (Apaf-1) and forms an important caspase cascade activator called apoptosome. This complex binds to caspase 9.
• The cytoplasm of normal cells contains proteins which block the activation of caspases and function as physiological inhibitors of apoptosis. Other mitochondrial proteins may enter the cytoplasm and neutralize the action of these cytoplasmic proteins and initiate caspase cascade.
• Also the proapoptotic proteins bind to and block the function of protective antiapoptotic proteins namely BCL2 and BCL-XL.

Mitochondrial injury ——> Activation of proapoptotic proteins (BAX, BAK) ——> Increased mitochondrial permeability——>

mitochondrial proteins leak into cytoplasm ——> initiation of caspase cascade ——-> Apoptosis

Extrinsic or death receptor pathway:

• This pathway is regulated by extracellular signals.
• Many cells express death receptors molecules on the plasma membrane that trigger apoptosis. Death receptors are members of the TNF (tumor necrosis factor) receptor family. The well known death receptors are the type 1 TNF receptors (TNFR1) and a related protein called Fas (CD95) and the binding ligand of Fas is Fas ligand (FasL).
• In extrinsic pathway, apoptosis is initiated when the death receptors present gets activated.
• Functions of extrinsic pathway: This pathway is involved in eliminating – self reactive lymphocytes thereby avoiding autoimmunity, virus infected cells and tumor cells.

2. Execution Phase:

The above mentioned two initiating pathways activates the initiator caspase-8. The initiator caspase activates another series of caspases called executioner caspases (casapse-3 and 6).

These executioner caspases induce fragmentation of nuclei. They also degrade and destroy components of nuclear matrix and cytoskeleton leading to fragmentation of involved cells.

Removal of apoptotic cells:

Now you may be wondering what happens to the apoptotic bodies that are formed as a result of fragmentation of cells. Well, these bodie are engulfed and removed by phagocytic cells mainly macrophages. This process of phagocytosis is so efficient that these dead cells and apoptotic bodies disappear within minutes.

Disorders associated with dysregulated Apoptosis:

Due to reduced Apoptosis:

• Cancer
• Autoimmune diseases

Due to increased Apoptosis:

• Neurodegenerative diseases (Alzheimer, Huntington, Parkinson disease)
• Death of virus infected cells: many viral infections, important being acquired immune deficiency syndrome.
• Ischemic injury (injury caused by restricted or reduced blood flow in a part of the body

Differences between Apoptosis and Necrosis:

REFERENCE:

Robbins and cotran (pathologic basis of diseases)

Pathology for undergraduates by Ramdas Nayak and Rakshatha Nayak